The 802.11a wireless local area network (WLAN) standard defines operations in the 5 GHz radio frequency band. In this frequency band, there are regulatory requirements, at least in Europe, to avoid transmissions in frequency bands or channels that are occupied by radar signals. Since these radar signals do not necessarily remain on a single channel, it might be necessary to cease normal data transmissions or data traffic on one channel and change to another channel so as not to disturb the radar. Another circumstance in which a channel switch may be necessary is if a channel has become subject to high interference. In this case, it is not necessary to cease transmission but the probability of receiving a transmission of data successfully is sufficiently low that the effect on the data output is very similar.
The so-called 802.11h draft standard specifies a method for channel switching, whereby a central control unit or controller in a network can indicate to the other wireless devices associated with it that it will change to another channel by a message transmitted either as part of the regular announcement (called the beacon in 802.11), or in a separate channel switch announcement which contains the message. This message causes a channel switch to be scheduled either immediately after the current transmission, or before the Nth subsequent beacon transmission with repeat transmissions of the channel switch message in each beacon leading up to the actual channel switch time.
The reason for delaying the channel switch in this manner is that a number of stations or devices in the network may be in so-called power-save or sleep mode and thereby unable to receive the messages. A graphical description of this so-called STA sleep functionality can be found in FIG. 1. According to the 802.11 protocol, these sleeping stations or devices are required to return to operating condition shortly before each expected beacon transmission time in order to check for pending traffic. Hence, the known method implies that the central controller repeats the message in subsequent beacons until all stations associated with the central controller are guaranteed to have awoken from power-save or sleep mode. This procedure is shown graphically in FIG. 2. A flow-chart of the known method is shown in FIG. 3.
A drawback of this known method is that, according to certain regulatory requirements in the standard, all normal data traffic on a channel must cease once radar signals have been detected on the channel (i.e. after the first channel switch announcement sent by the central controller). The requirements allow sending the beacons on the old channel enough times to ensure that all connected devices have heard the announcement, typically 10 seconds total with a maximum of 20 ms control traffic accumulated. This means that all the currently active devices or terminals in the network must cease sending data traffic until all of the necessary beacon messages have been sent (with a typical spacing between beacons of 0.1 s and a typical sleep duration in the range of some seconds), before the channel switch occurs and the terminals can resume operation on the new channel. This causes an undesirable disruption in the data traffic that is particularly disastrous for data traffic with so called quality of service (QoS) requirements on maximum delay, such as video or voice traffic.
A second drawback of the known method is that if the channel switch is made too quickly, with too few repeats of the announcements, there might be terminals or devices in the network that are left without a central control unit on the old channel. This means that the terminal or device must start searching through a list of frequencies or channels to try to find the old central controller or an alternative one, which is time consuming. In QoS terms, this will be a lost link.
A third drawback of the known method is that if the announcements are repeated many times, there will be a loss of traffic during the times when the announcements are made which will also lead to a lost link.